This disclosure relates to biodegradable aliphatic-aromatic copolyester compositions, and methods of manufacture of the copolyesters and compositions. These copolyesters and compositions are useful as molded or extruded plastic objects, films, and fibers.
Aliphatic-aromatic copolyesters are often used for the manufacture of biodegradable disposable articles. For example, U.S. Pat. No. 6,020,393 discloses a branched, random aliphatic-aromatic copolyester suitable for forming into biodegradable disposable articles, including poly(butylene-co-adipate terephthalate) (PBAT). U.S. Pat. No. 6,201,034 discloses processes for preparing PBAT by reacting dimethyl terephthalate (DMT) or terephthalic acid (TPA) and adipic acid (AA) with butanediol (BDO). The biodegradability is induced by the incorporation of adipic acid in poly(butylene terephthalate) (PBT). The polymer thus made has a typical melting point (Tm) of about 109° C., and a glass transition temperature (Tg) between −25 to −30° C. The polymerization is conducted using a transesterification (TE) catalyst such as a titanium or tin compound.
The present inventors have observed that biodegradable aliphatic-aromatic copolyester product obtained in such a reaction was discolored, often ranging from pink to red in color. This presents a problem in that the aesthetic appearance of a non-white polymer product is an obstacle to employing the polymer in end-uses where the discoloration is apparent and cannot be readily overcome or masked with pigments, whitening agents or fillers. It has further been found that it is difficult to achieve a white color while at the same time maintaining or improving upon other desirable properties of biodegradable aliphatic-aromatic copolyesters such as PBAT. For at least the foregoing reasons, there remains a long unfelt need to develop processes that produce useful biodegradable aliphatic-aromatic copolyesters.